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The 4,4,4-Trifluoroacetoacetic Acid Keto−Enol System in Aqueous Solution. Generation of the Enol by Hydration of Trifluoroacetylketene

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journal contribution
posted on 25.08.1999, 00:00 by Y. Chiang, A. J. Kresge, Q. Meng, Y. Morita, Y. Yamamoto
Trifluoroacetylketene was generated in aqueous solution by flash photolysis of 2,2-dimethyl-6-trifluoromethyl-4H-1,3-dioxin-4-one and its spiro analogue, 4-trifluoromethyl-1,5-dioxaspiro[5.5]undec-3-en-2-one, and rates of hydration of the ketene to 4,4,4-trifluoroacetoacetic acid enol as well as subsequent ketonization of the enol were measured in this solvent across the acidity range [H+] = 10-1−10-12 M. Analysis of the rate profile produced by these data provides the acidity constants pQa,E = 1.85 for the carboxylic acid group of the enol and p = 9.95 for its enolic hydroxyl group, which make these groups 2 and 3 orders of magnitude more acidic, respectively, than the corresponding groups in the parent unfluorinated acetoacetic acid enol. Rates of enolization of the keto group of 4,4,4-trifluoroacetoacetic acid were also measured, by bromine scavenging, and these, together with a value of the equilibrium constant for hydration of the keto group to its gem-diol derivative based upon a free energy relationship, Kh = 2900, provide an estimate of the keto−enol equilibrium constant for this system:  pKE = 0.28. This is greater, by 2 orders of magnitude, than the keto−enol equilibrium constant for the unfluorinated acetoacetic acid.